An NAT (Network Address Translation) router has widely been known. The NAT router is interposed between the Internet and a local network. The NAT router performs processing for translating an address between a global address (a global IP address and a port number) and a local address (a local IP address and a port number). For example, in order to perform a data communication between a first terminal device belonging to a first local network and a second terminal device belonging to a second local network, a global address of the first terminal device and a global address of the second terminal device are usually used. In this case, a first NAT router assigned to the first local network translates an address of data communicated between the first terminal device and the Internet (i.e., the second terminal device). A second NAT router assigned to the second local network also translates an address of data communicated between the second terminal device and the Internet (i.e., the first terminal device).